A high-performance, single-focus or zoom lens unit for photograph cameras, broadcasting cameras, etc. generally has about 5-40 lenses in a lens barrel. There are also wide-angle lenses for wide image, which have large incident angles of light beams in peripheral portions. Also, a lens having a small radius of curvature to its effective diameter is sometimes disposed in a light path for optical design. Formed on optical members such as these lenses are multi-layer, anti-reflection coatings comprising a combination of dielectric films having different refractive indices for utilizing interference effects with dielectric film thickness of ½λ or ¼λ to a center wavelength λ.
JP 2009-193029 A discloses an anti-reflection coating comprising a first layer of alumina having an optical thickness of 97.0-181.0 nm, a second layer having an optical thickness of 124.0-168.5 nm and a refractive index of 1.33-1.50, which is made of at least one selected from MgF2, SiO2 and Al2O3, and a third layer of porous silica having an optical thickness of 112.5-169.5 nm formed in this order on a substrate having a refractive index of 1.60-1.93 to light in a wavelength range of 400-700 nm, the first and second layers being formed by a vacuum vapor deposition method, and the third layer being formed by a sol-gel method. JP 2009-193029 A describes that this anti-reflection coating has excellent anti-reflection characteristics, with alumina used in the first layer preventing the weathering of the substrate surface.
The anti-reflection coating described in JP 2009-193029 A comprises a porous silica layer having a refractive index of 1.07-1.18 as the third layer, as is clear from Examples. However, it is difficult to stably produce a porous silica layer having extremely high porosity for such a low refractive index. When the porous silica layer has a refractive index of more than 1.18, an optimum anti-reflection coating cannot be obtained by the structure described in JP 2009-193029 A.
JP 2009-210733 A discloses an anti-reflection coating comprising a first layer of alumina having an optical thickness of 25.0-250.0 nm, a second layer having an optical thickness of 100.0-145.0 nm and a refractive index of 1.40-1.50, which is made of at least one selected from MgF2, SiO2 and Al2O3, and a third layer of porous silica having an optical thickness of 100.0-140.0 nm formed in this order on a substrate having a refractive index of 1.53 or more and less than 1.60 to light in a wavelength range of 400-700 nm, the first and second layers being formed by a vacuum vapor deposition method, and the third layer being formed by a sol-gel method. JP 2009-210733 A describes that this anti-reflection coating has excellent anti-reflection characteristics, with alumina used in the first layer preventing the weathering of the substrate surface. However, because this anti-reflection coating fails to exhibit optimum anti-reflection performance when formed on a substrate having a refractive index of 1.60 or more, further improvement is desired.
JP 10-319209 A discloses a method for producing an anti-reflection coating on a substrate, which comprises the steps of forming a first anti-reflection layer of a first material by a wet or dry process, and forming a second anti-reflection layer of a second material on the first anti-reflection layer by a wet process. It describes that one or more anti-reflection layers may be additionally formed under the first anti-reflection layer by a wet or dry process, the dry process being selected from vacuum vapor deposition, sputtering and CVD, and the wet process comprises a sol-gel process. It further describes that such method provides an anti-reflection coating having high performance such as low reflectance, and wide wavelength and angle ranges of anti-reflection, particularly in an ultraviolet range, with a small number of layers.
JP 10-319209 A discloses an anti-reflection coating having two-layer structure comprising a first layer of SiO2 (dry process) and a second layer of porous SiO2 (sol-gel method), and an anti-reflection coating having a three-layer structure comprising a first layer of LaF3, NdF3 or GdF3 (dry process), a second layer of SiO2 (wet or dry process), and a third layer of porous SiO2 (sol-gel method). However, because this anti-reflection coating has insufficient weathering-preventing effects, further improvement is desired.
Further, when these anti-reflection coatings are formed on lens substrates having maximum inclination angles of 30° or more, their anti-reflection performance is extremely poor in portions having large inclination angles, failing to obtain sufficient anti-reflection effects. Accordingly, the development of anti-reflection coatings suitable for lens surfaces having such large inclination angles is desired.
JP 2012-18286 A discloses an optical member comprising an anti-reflection coating comprising first and second layers formed on a substrate by a dry process, and a third layer of silica aerogel formed by a wet process, to light in a wavelength range of 550 nm, the refractive index of the substrate being higher than that of the first layer and 1.9 or less, the refractive index A of the third layer being in a range of 1.18-1.32, the first to third layers having refractive indices decreasing in this order from the first layer, and the optical thickness Y of the second layer meeting the conditions of −750A+945≦Y≦−750A+1020, and 20≦Y≦120. JP 2012-18286 A further describes that to have high adhesion, and to prevent the weathering of optical glass substrate surfaces, a material forming the first layer is preferably Al2O3. It has been found, however, that when this optical member is stored under high-temperature, high-humidity conditions for a long period of time, the anti-reflection coating suffers fogging, resulting in deteriorated spectral reflection characteristics.